1. Field of the Invention
The present invention relates to a method for manufacturing a sintered body with high density from powder.
2. Description of the Prior Arts
The method described below is generally known as a method for molding powder by casting.
Ceramic powders are dispersed in water to be turned into slurry. Deflocculating agent is added to the slurry and slurry properties are adjusted. The slurry with the adjusted properties is cast into a gypsum mold and solidified into a compact by letting the gypsum mold absorb a part of water contained in the slurry. The compact is dried by air drying or by heat drying to be turned into compact which can be provided to a sintering process. In this method, a heat evaporation is used as means for removing dispersion medium in case the dispersion medium is water. The dispersion medium is removed by a thermal decomposition when the dispersion medium is paraffin. However, the removal of the dispersion medium by either heat evaporation method or thermal decomposition method is disadvantageous in that both methods require lots of time.
Apart from the above-mentioned methods, Mr. Nishio and others, the inventors in the present application, disclosed the following in a Japanese Patent Application No. 100433/85 (a Japanese Patent Application Laid Open (KOKAI) No. 261274/86): metal or ceramics powder are dispersed in various sorts of alcohol to be turned into slurry. The slurry is cast into a gypsum mold and solidified into a compact by letting the gypsum mold absorb a part of dispersion medium. The dispersion medium contained in the compact is extracted and removed by supercritical carbon dioxide. In this patent application, however, only the method for removing the dispersion medium contained in the compact is disclosed. In the meantime, a hot isostatic pressing process is known as a method for turning into a high density sintered body, the porous compacts obtained by various sorts of methods for molding powders such as mold casting, plastic forming, press forming or the like. In the hot isostatic pressing process, a compact is sintered and is pressed isostatically by gas under conditions: temperatures of approximately from 500 to 2,200.degree. C and pressures of approximately from 100 to 3,000 kg/cm.sup.2 G. At the time of sintering under these conditions, it is necessary that a pore sealing process is applied in advance to the compact so that a gas pressure can spread over the surface of the compact without letting the gas enter into the pores of the compact. The methods described in a Japanese Examined Patent Publication No. 35870/84 and in a West German Patent No. 3,403,917 are introduced as methods of the pore sealing process.
The following is disclosed in the Japanese Examined Patent Publication No. 35870/84: an inner porous layer comprising low melting glass, low melting glass forming substance or high melting metallic substance is formed on the surface of a compact of silicon nitride, and the inner porous layer can turn into an impermeable film layer to high pressure gas at 1,300 to 1,600.degree. C. i.e. at a temperature or less of sintering the silicon nitride. An outer porous layer, which can turn into an impermeable film layer comprising a low melting glass or low melting glass forming substance is formed on the surface of the inner porous layer, and the outer porous layer can turn into an impermeable film layer to the high pressure gas at a temperature or less at which the inner porous layer so does. Subsequently, after a degassing treatment is applied to the porous compact covered with the inner porous layer and the outer porous layer, a heat treatment comprising a first and a second heat treatment will be applied to the compact. The first heat treatment is carried out at a temperature which is necessary for the formation of the impermeable film layer from the outer porous layer, but enables the inner porous layer to maintain its porosity. Successively, the second heat treatment is carried out until a temperature is raised upto the temperature necessary for the formation of the impermeable film layer from the inner porous layer. After the heat treatment, a hot isostatic pressing process is applied to the porous compact at a temperature of from 1,700.degree. to 1,800.degree. C. and at a pressure of from 2,000 to 3,000 atm. In this method, however, to form the porous layer on the surface of the porous compact, slurry is made by dispersing powders in water, the porous compact is immersed in the slurry and thereafter, dried. This drying operation, which takes lots of time, needs to be repeated twice so as to form two layers of the inner and outer porous portions.
A density increasing method comprising the following steps is disclosed in the West German Patent No. 3,403,917 as a method for molding an intricate shape compact porous ceramics:
(a) A porous ceramic compact preliminarily formed is immersed in a suspension to form a first capsular layer on the surface of the porous compact, the suspension being prepared by dispersing a substance containing no sintering auxiliary in a solvent;
(b) The solvent in the first capsular layer is evaporated;
(c) Furthermore, the compact covered with the first capsular layer is immersed in a suspension to form a second capsular layer on the surface of the first capsular layer, the suspension being prepared by dispersing a substance containing one or more sorts of auxiliaries in a solvent;
(d) The solvent in the second capsular layer is evaporated;
(e) The compact, on the surface of which the first and the second capsular layers have been formed, is sintered in an atmosphere of protective gas;
(f) A hot isostatic pressing process is applied to the sintered compact; and
(g) The first and the second capsular layers are removed mechanically.
Even in this method, however, an operation for evaporating and removing the solvent has to be repeated twice because the two porous layers are formed on the surface of the porous compact.